With the development of a three-dimensional fine processing technique in recent years, attracting attention are systems that comprise fluid elements, such as a fine flow channel, a pump, and a valve, and a sensor integrated on a substrate, such as glass or silicon, to conduct chemical analysis on the substrate. Such a system is called a miniaturized analysis system, a μ-TAS (Micro Total Analysis System), or a Lab on a Chip. The miniaturization of a chemical analysis system enables a decrease of noneffective space volume and a remarkable decrease in the sample size, as well as a reduction of the analysis time and a decrease in power consumption of the entire system. Further, the miniaturization promises to lower the price of the system. Furthermore, the μ-TAS is a promising system for use in medical services, such as home medical care and bed-side monitoring, and biological techniques, such as DNA analysis and proteomic analysis.
Japanese Patent Application Laid-Open No. 10-337173 discloses a microreactor, which is suitable for conducting a sequence of biochemical experimental steps comprising mixing and reacting solutions, determination and analysis, and separation, by utilizing a combination of several cells. FIG. 6 illustrates schematically a concept of microreactor 601. Microreactor 601 has an isolated reaction chamber sealed with a flat plate on a silicon substrate. This microreactor has reservoir cell 602, mixing cell 603, reaction cell 604, detection cell 605, and separation cell 606 in combination. By providing more than one such a reactor on a substrate, many biochemical reactions can be allowed to proceed simultaneously and in parallel. Not only the analysis, but material synthesis, such as protein synthesis, can be conducted in a reactor.
In Jr-Hung Tsai and Liwei Lin, “A Thermal Bubble Actuated Micro Ejection orifice-Diffuser Pump”, Proceedings of 2001 IEEE Micro Electromechanical Systems Workshop, 2001, pp. 409 to 412, a device is disclosed in which a liquid is heated by a heater to form a bubble, so that the liquid is transported by using the expansion and shrinkage of the bubble. FIGS. 7A and 7B show the principle of this device. In this device, a heat generating element 703 is formed in a chamber 702. Tapered flow channels 706 and 705 are formed at an inlet 707 and an outlet 704 communicating with the chamber 702. A bubble 701 is generated in the chamber by applying a voltage to the heat generating element 703. The generated bubble expands for a certain time period, then shrinks, and disappears.
At the time of expansion of the bubble 701, a liquid in the chamber 702 flows out of the chamber by a force applied to the liquid by the expansion of the bubble. A difference in flow channel resistance occurs between the inlet 707 and the outlet 704 due to the tapered shapes of the flow channels 706 and 705. Therefore, the flow rate at which the liquid flows out through the outlet 704 is higher than that at which the liquid flows out through the inlet 707 (FIG. 7A).
At the time of shrinkage of the bubble 701, the liquids at the outlet and inlet sides flow into the chamber. In this case, the flow rate at which the liquid flows in through the inlet 707 is higher than that at which the liquid flows in through the outlet 704 (FIG. 7B), in contrast with the expansion case.
The heat generating element 703 is repeatedly driven to cause the bubble 701 to repeat expanding and shrinking. The liquid is thereby transported from the inlet 707 side to the outlet 704 side (the direction from right to left as viewed in FIGS. 7A and 7B).
Conventionally, in a case where a microreactor, such as the one disclosed in Japanese Patent Application Laid-Open No. 10-337173 and shown in FIG. 6, is used, a silicone tube, for example, is connected to the reservoir cell 602 and a liquid sample is introduced into the reactor by using a syringe pump or the like. In such a case, the syringe pump is required outside the microreactor, disadvantageously increasing the cost and the size of the entire system. In a case where a liquid sample is applied dropwise in the reservoir by using a dispenser or the like, a considerably large device is also required outside the microreactor.
Also, in a microreactor such as that shown in FIG. 6, it is possible that a liquid mixed in the mixing cell 603 or a liquid caused to react in the reaction cell 604 will flow backward to the reservoir cell 602, and a stable chemical reaction will not be performed. As a method of preventing such a backward flow, it is possible to form a microvalve in the microreactor. However, a considerably large number of steps are required to form a microvalve, resulting in an undesirable increase in the manufacturing cost. Moreover, since the valve is opened and closed a larger number of times, the opening/closing performance and sealing characteristics of the valve deteriorate with time and the useful life of the microreactor is reduced.
In the liquid transport device shown in FIGS. 7A and 7B, outlet 704 and inlet 707 are in liquid communication, and it is possible that the liquid at the outlet 704 side can flow backward toward the inlet 707 side. In particular, when the heat generating element is not driven, a diffusion occurs due to the liquid communication between outlet 704 and inlet 707, resulting in a mixing of the liquid at the outlet 704 side and the liquid at the inlet 707 side. In order to prevent this, there is also a need to form a microvalve.